The answer depends on how you define Gynecomastia.
One common definition is: any enlargement, feminization or contouring of the male breast.
Obviously when such a general definition is used you could call any enlargement of the breast Gynecomastia.
Usually, the enlargement is both fatty tissue and glandular tissue. Rarely is it exclusively one or the other.
If a person is generally obese, weight loss might have a significant effect on breast size. The key word is might.
The breast area seems to be one of the first places many young men deposit fat and one of the last places where they can lose it. Weight loss even to a dangerous extent sometimes has little or no effect on the breast size.
Likewise, exercise has little effect if any. And the steroids often used by body builders are a common element in many cases of Gynecomastia.
While Gynecomastia is not usually harmful in any physical way, It can be an important symptom of several serious medical problems so it is always wise to ask your Doctor.
The effects of Gynecomastia are upon the self image and attitude of the young man who has the condition. These are not physical problems but Psychological issues. And Yes, The problems can be severe. They are essentially the same whether the breasts are mostly fatty, mostly glandular, or both. Many people, on these grounds, do not differentiate between the various types.
Surgeries performed to excise glandular tissue only or Liposuction for fat removal only have a relatively high rate of dissatisfaction. A wiser choice would seem to be trusting the Surgeon to exercise their own judgement as the procedure progresses. Obviously this would be more expensive, But the satisfaction rate is also much higher.
Many young men simply adapt and get on with their lives. In fact, the men of my generation had no choice since the surgery to reduce men's breasts is comparatively new.
Good luck!